1. Field of the Invention
The present invention relates in general to an ink-jet printing head, and more particularly to the construction of a large-sized ink-jet printing head having a large number of nozzles arranged in at least one row.
2. Discussion of Related Art
A prior art ink-jet printing head of on-demand type, as disclosed in JP-A-2002-36545 and U.S. Patent Application Publication U.S. 2002/0024567 A1, for example, includes a cavity unit consisting of a plurality of plates superposed on each other so as to define ink delivery passages. These plates include a nozzle plate having a plurality of nozzles, a base plate partially defining pressure chambers corresponding to the respective nozzles, and manifold plates partially defining manifold chambers which communicate with an ink supply source and the above-indicated pressure chambers. The ink-jet printing head further includes a piezoelectric actuator which includes piezoelectric ceramic plates, and internal electrodes in the form of common electrodes and arrays of individual electrodes formed on the piezoelectric ceramic plates such that the common electrodes and the individual electrode arrays are alternately superposed on each other. The piezoelectric actuator and the cavity unit are bonded together such that active portions existing between the common electrode and the respective individual electrode are aligned with the respective pressure chambers.
In an ordinary ink-jet printer known in the art, a printing operation is performed by an ink-jet printing head in a direction of width of a recording medium such as a sheet of paper, which direction is perpendicular to a direction of feeding of the recording medium. The direction of width and the direction of feeding of the paper sheet will be respectively referred to as “primary scanning direction” and “secondary scanning direction” where appropriate. The printing operation is performed such that rows of the nozzles of the ink-jet printing head are parallel to the direction of feeding of the paper sheet (the secondary scanning direction). In this arrangement, images can be printed during each one movement of the carriage in the primary scanning direction, in the corresponding area of the paper sheet whose dimension in the secondary scanning direction is substantially equal to the length of each row of the nozzles. For example, the ink-jet printing head has a plurality of parallel rows of nozzles, each of which has a length of one inch (25.4 mm) and consists of 72 nozzles, and the nozzles in the parallel rows are arranged such that the nozzles of one row and the nozzles of the adjacent row are positioned in a zigzag pattern. In this case, the area in which a printing operation is performed on the paper sheet during one movement of the ink-jet printing head in the primary scanning direction has a dimension of one inch in the secondary scanning direction. This dimension may be referred to as “maximum printable height” per one movement of the ink-jet printing head in the primary scanning direction.
To meet recent demands for an increased printing speed and an improved quality of printed images, there has been a need for increasing the length of the rows of the nozzles to about two inches, for instance, by increasing the number of the nozzles in each row while maintaining the spacing pitch of the nozzles (dot-to-dot distance) in the secondary scanning direction.
On the other hand, each manifold chamber formed in the cavity unit, between the nozzles in the corresponding row and the ink supply source, is provided to store a suitable volume of an ink supplied from the ink supply source, and is arranged to re-fill the pressure chambers with the ink when the actuator is operated according to printing commands, to deliver the ink from the selected pressure chambers to the corresponding nozzles so that droplets of the ink are jetted from the nozzles onto the paper sheet. Where the cavity unit has a relatively large number of nozzles arranged in each row, the ratio of the volume of the corresponding manifold chamber to the entire volume of the cavity unit must be increased for the reason described below.
Namely, the length of each manifold chamber must be increased with an increase in the number of the nozzles in the corresponding row. However, a mere increase in the length of the manifold chamber in the direction of extension of the row of the nozzles will cause the following problems.
When the ink flows through the manifold chamber toward the nozzle at one end of the row of the nozzles which is furthest from the portion of the manifold chamber at which the manifold chamber communicates with the ink supply source, the rate or amount of flow of the ink tends to decrease in the direction of the flow due to a resistance to the flow of the ink mass in contact with the wall surfaces of the manifold chamber.
To prevent a decrease in the pressure of the ink mass flowing toward the ends of the manifold chamber, it is required to increase the volume of the manifold chamber to the entire volume of the cavity unit, by increasing the width dimension of the manifold chamber in the direction of width of the manifold plates (perpendicular to the direction of extension of the rows of the nozzles), so that the external dimensions of the cavity unit as viewed in the plane parallel to the manifold plates are accordingly increased, contrary to a need to reduce the size of the cavity unit.
When the ink in each pressure chamber is instantaneously pressurized upon activation of the corresponding portion of the actuator, a pressure wave of the ink in the pressure chamber includes a reverse component propagating in a direction toward the manifold chamber, as well as a forward component propagating in a direction toward the corresponding nozzle. In this respect, a change in the configuration of each manifold chamber, in particular, its longitudinal dimension, requires corresponding changes in the nominal magnitude and timing of operation of the actuator, and in the nominal waveform of the pressure wave indicated above. For instance, a cavity unit whose rows of nozzles has a length of one inch (having a comparatively small number of nozzles) and a cavity unit whose rows of nozzles have a length of two inches (having a comparatively large number of nozzles) have different nominal pressure waves of the ink in the pressure chambers, and should therefore have different designs in basic arrangements such as different magnitudes and timings of operation of the actuator, leading to problems of an increase in the required cost of development of the cavity units and an increase in the time required to complete the ink-jet printing heads as commercial products.
For increasing the length of each row of the nozzles with an increase in the number of the nozzles in each row, the nozzles and pressure chambers can be formed in the plates of the cavity unit, with the nominal spacing pitches or distances with high accuracy, irrespective of the number of the nozzles and pressure chambers, where the nozzles and pressure chambers are formed by laser machining or etching operations in those plates formed of a metallic or synthetic material.
For providing each piezoelectric ceramic plate of the piezoelectric actuator with the active portions corresponding to the respective nozzles, on the other hand, the length of the piezoelectric ceramic plate should necessarily be increased with an increase in the number of the nozzles.
As known in the art, the piezoelectric actuator is fabricated by pressing and then firing a laminar structure wherein piezoelectric ceramic plates each having the common electrode formed thereon in a predetermined pattern and piezoelectric ceramic plates each having the individual electrodes formed in a predetermined pattern are alternately superposed on each other. Generally, the dimensions of the piezoelectric ceramic plates in the directions of length, width and thickness are reduced due to shrinkage of the plates as a result of a firing operation. In particular, the amount of shrinkage of the piezoelectric ceramic plates in the direction of length (perpendicular to the direction of extension of the rows of the nozzles) is considerably large. The spacing distance between the adjacent individual electrodes in the direction of length of the piezoelectric plates is determined with the above-indicated amount of shrinkage (shrinkage ratio) taken into account.
In the presence of variations in the fabrication of the piezoelectric ceramic plates, such as variations in the dimensional accuracy and firing temperature, however, it becomes more and more difficult to match the spacing distance between the adjacent individual electrodes formed on the fired piezoelectric ceramic plates, with the spacing distance of the adjacent pressure chambers, as the length of the piezoelectric ceramic plates is increased.